Make it possible for all idea portal users to create ideas via Slack integraiton Merged

At the moment, it's only possible for contributors of Aha to submit ideas inside Slack. This doesn't make much sense product owners / contributors all spend a lot of time in the browser version of AHa.

To make the Slack integration actually useful, it should be possible for anyone in the organisation to submit ideas from within Slack. It's for this user group (the people who doesn't spend their daily work day inside Aha) that an integration makes sense, so they don't have to leave their favourite tool to submit an idea.

This would require that portal users can sign in and submit ideas directly from within Slack.